Packed Bed Scrubber Sizing

Basis:-
Absorption of Formaldehyde from Formaldehyde + Air Mixture
Scrubbing Media is Sodium Sulphide, Na2S
Isothermal Absorption no temp. rise during absorption

Process Data
T Scrubber Operating Temp. 70
P Oeperating Pressure 1
760
HCHO Volume % in inlet gas 4
Pipe Diameter 450
Assuming Inlet Velocity 20
Inlet Vol. Flow Rate of Gas 11445.3
Molecular Wt. of Formaldehyde 30
Molecular Wt. of Air 29
R Universal Gas Constant 0.082
Absorption required 95
0.95
Nomenculture
Gs Molar Flow rate of Air
G1 Molar Flow of Inlet Gas Mixture
y1 Mole Fraction of HCHO in Inlet Gas
Y1 Kmol of HCHO/ Kmol Air in Inlet Gas
G2 Molar Flow of Outlet Gas Mixture
y2 Mole Fraction of HCHO in Outlet Gas
Y2 Kmol of HCHO/ Kmol Air in Outlet Gas
x2 Mole Fraction of HCHO in Inlet Water
X2 Kmol of HCHO/ Kmol Water in Inlet Liquid
L2=Ls Molar Flow Rate of Water at Inlet
L1 Molar Flow Rate of Water at Outlet
x1 Mole Fraction of HCHO in Outlet Water
X1 Kmol of HCHO/ Kmol Water in Outlet Liquid
Lsm Minimum Amount of Solvent Required
X1m Value of X1 corrospond to Minimum Amount of Solvent Req
Calculation
ρA Density of Air 1.031
Gs Molar Flow Rate of Air 406.93
x2 Mole Fraction of HCHO in Inlet Liquid 0
X2 Kmol of HCHO/ Kmol Water in Inlet Liquid 0.000
y1 Mole Fraction of HCHO in Inlet Gas 0.04
Y1 Kmol of HCHO/ Kmol Air in Inlet Gas 0.04167
y2 Mole Fraction of HCHO in Outlet Gas 0.002
Y2 Kmol of HCHO/ Kmol Air in Outlet Gas 0.00200
Mole Fraction of Air in Outlet Gas 0.998
Minimum Amount of Solvent required for desired absorption
Antoine's co. A 7.1561
HCHO B 957.24
C 243.01
Pv Vapour Pressure of HCHO @ 70 °C 250.29
m Pv/ P 0.33

Ls=MWR= Minimum Wetting Rate required to form Liquid Film all over Packing.
MWR can be found out based on Tower Diameter which is yet not known
So this is required Trial & Error method for estimation of MWR
Minimum 1 M3/hr of Flow Rate is required for use of Centrifugal Pump for facilate absorption

ρL Density of Sol. @ atm. temp. 1800

Mw Molecular Wt. of Sol. 78
Ls Minimum Solvent Flow Rate 4
92.31
7200

Absorption Column Tower Diameter required at Top based on Inlet Condition

Lw Mass Velocity of Solvent/ Water entering Tower
Lw= (1000/3600)/(0.785*D2)
Lw=0.2778/(0.785*D2)
G2 Molar Flow of Outlet Gas Mixture 407.75
G2=Gs(1+Y2) 3.2848391977

Mav Avg. Molecular Wt. of Gas Mixture outlet 29.002

Mav= Mole fraction* Molecular Wt.

Gw Gw=(G2*Mav)/(0.785*D2)
Gw=0.462/(0.785* D2)

ρg Density of Outlet Gas Mixture 1.031

FLG Lw/Gw 0.0846

(ρg/ρL)^1/2 0.0239
FLG 0.0020
From Fig. 9.3 Find Kf Value

Kf From Chart of FLG vs K at max. flooding 0.2

Considering @ 66 % of Flooding

F Flooding Considering 66
K At 66% Flooding 0.08712

From Fig. 9.3 Find Pressure Drop at 66% Flooding

 Pressure Drop 60

Ψ Density of Na2s 1.8

g Gravitational Accelration 9.81
µL Viscosity of Na2s @ 20 °C 1
0.001

Packing Detail
Select Packing Metal Pall Ring
Packing size 50
Fp Packing Factor 66

Gw Mass Velocity of Gas through Tower 3.654

At Tower Area required at Top 0.8989

Dt Tower Diameter required at Top 1.0701175837
1070.1175837273
Say 1100
Tower Diameter/ Packing Size 22
Recommended > 10 so satisfactory
Wetting Liquid Rate 4.211
Recommended MWR required is 4 m3/m2*hr for random packing
If We keep 1 m3/hr Rate MWR will fall below Rocommended MWR Value
Let's increse Solvent Rate to get MWR 4 m3/m2*hr
Liquid mass flow Rate per m2 of tower 7580.1
Kister has also suggested keep MWR of circulation 5 M3/m2*Hr

Based on Morris & Jackson Minimum Solvent Required

Lmin. Minimum Solvent Required = MWR* at
MWR for Ring <75 mm is 0.08
at For 50 mm SS Pall Ring 115
Lmin. Minimum Solvent Required 9.2
14887
L2 190.85

Cross Checking of Liquid Rate based on this method

FLG 0.030
From Figure 9.3 FLG Vs. K at Flooding
Kf From Fig. 9.3 0.17
Let the Actual Velocity of Gas = 66 % of Flooding Velocity

F Flooding Considering 66
K At 66% Flooding 0.074
From Fig. 9.3 Find Pressure Drop at 66% Flooding
Pressure Drop 40
 Gw Mass Velocity of Gas through Tower 3.369
At Tower Area required at Top 0.9750
Dt Tower Diameter required at Top 1.11
1114.49
Say 1150

Lm Liquid Rate 15267.6

8.5
Hence it is satisfied the condition based on morries & Jackson calculation

Absorption Column Tower Diameter required at Bottom based on Outlet Condition

Mav Avg. Molecular Wt. of Gas- Vapour Mixture 29.04

Mav= y1*m.w of HCHO+ (1-ye) m.w of air

ρg Density of Outlet Gas Mixture 1.032

HCHO Balance
Gs(Y1-Y2)=Ls(X1-X2)
X1 Kmol of HCHO/ Kmol Water in Outlet Liquid 0.084567
x1 Mole Fraction of HCHO in Outlet Water 0.077973
ρL Density of Solution leaving Tower 1800.0
L1 Molar Flow Rate of Water at Outlet 206.9926416931
L1=Ls(1+X1)

Mav Avg. Molecular Wt. of Solution leaving Tower 74.26

L1w Mass Velocity of Solvent/ Water leaving Tower
L1w= (L1*Mav/3600)/(0.785*D2)
L1w=4.25/(0.785*D2)

G1 G1=Gs(1+Y1) 423.88
3.42
G1w Mass Velocity of Gas leaving Tower
G1w= (G1*Mav gas/3600)/(0.785*D2)
G1w=3.42/(0.785*D2)

FLG Lw/Gw 1.2427

(ρg/ρL)^1/2 0.0240
FLG 0.0298
From Figure 9.3 FLG Vs. K at Flooding
Kf From Fig. 9.3 0.175
Let the Actual Velocity of Gas = 66 % of Flooding Velocity

F Flooding Considering 66
K At 66% Flooding 0.076
From Fig. 9.3 Find Pressure Drop at 66% Flooding
Pressure Drop 55

Gw Mass Velocity of Gas through Tower 4.589

4.4
At Tower Area required at Top 0.7451
Dt Tower Diameter required at Top 0.97
974.28
Say, Tower Diameter required 1000

Packing Height req.

m Gm/Lm Avg. Gas Mass Velocity 0.688

NoG Nos. of Gas Phase Transfer Unit 6.21

HETP Assume for Random Packing 0.6
H Height Of Packing required 3.72
3
Packed Tower Overall Pressure Drop

∆P Overall Pressure Loss 165

Tower Diamter ∆P/ Mtr

1000 40 mmwc
 Summury of Calculation

Tower Data
Tower Diameter 1200 mm
Packing Height 3000 mm
Tower total Pr. Drop 120 mmwc
Velocity inside Tower 4.4 m/sec
°C
Atm Pump Requirement
mmhg Flow Rate 6 CMH
% Head 10 Mtr
mm
m/s Blower Requirement
m3/hr Flow Rate 12000 CMH
g/mol Static Pressure 600 MMWC
g/mol 6000 Pa
m3 atm/ kmol k Estimated Motor 20 KW
% 27 HP
Say 30 HP

Kmol/hr
Kmol/hr

Kmol/hr

Kmol/hr
Kmol/hr

Kmol/hr
Kmol/hr

Kg/m3
Kmol/hr
 mmhg

absorption

Kg/m3
g/mol
m3/hr
Kmol/hr
Kg/hr

Kg/ m2 sec
Kmol/hr
Kg/sec

Kg/ Kmol

Kg/ m2 sec

Kg/m3

%
 mmwc/m packing

m/s2
Cp
Kg/ m sec

SS-316
mm
m-1

Kg/ m2 sec

m2
m/s2
mm
mm

m3/m2 hr
ng
WR Value

Kg/ hr m2

m2/ hr
m2/m3
m3/ m2 hr
Kg/hr
Kmol/hr

mmwc/m packing
 Kg/ m2 sec
m2
m/s2
mm
mm

Kg/ m2 hr
M3/ m2 hr

on

Kg/ Kmol

Kg/m3

Kg/m3
Kmol/hr

Kg/ kmol

Kg/ m2 sec

Kmol/hr
Kg/sec

Kg/ m2 sec

%
mmwc/m packing

Kg/ m2 sec
m/s
m2
m
mm
mm

Nos.

mmwc

Total ∆P
120 mmwc